The field of the invention is coating processes and coated articles from polyamides using fluidized bed, flame spraying and electrostatic coating. The invention is particularly concerned with an improvement in polyamide coatings using an intermediate layer of polybutadiene from an aqueous dispersion.
The state of the art of coating a metal substrate with plasticized polylaurolactam and polyesters by fluidized bed, flame spraying and electrostatic coating methods may be ascertained by reference to Assignee's U.S. Pat. Nos. 3,900,607 and 4,011,188, the disclosures of which are incorporated herein.
The preparation of polyamide powders is known from Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Vol. 16 (1968) pp. 88-105, particularly pp. 101 and 102 and U.S. Pat. No. 4,143,025. The powders are obtained by precipitating the polyamide from solutions or by grinding the polyamide granulate, preferably at low temperatures and under an inert atmosphere.
The state of the art of polybutadiene may be ascertained by reference to Kirk-Othmer, 2nd Edition, Vol. 7 (1965), pp. 73-75 and Kirk-Othmer, 3rd Edition, Vol. 4 (1978), pp. 313-337, the disclosures of which are incorporated herein.
In order to achieve high resistance to corrosion and to chemicals, polyamide coatings applied by sintering processes require that they firmly adhere to the base. The term sintering processes includes fluidized bed, flame spraying and electrostatic coating.
Many different substances have been used as adhesive intermediate layers or base coats for polyamide coating and the state of the art of these intermediate layers may be ascertained by reference to U.S. Pat. Nos. 3,203,822 and 3,264,131 the disclosures of which are incorporated herein; British Pat. No. 950,502; W. German Pat. No. 20 03 702 and published Japanese application No. Sho-47 (1972) 4951.
U.S. Pat. No. 3,264,131 discloses the use of heat-setting phenolic resins, possibly together with thermoplastic resins as intermediate layers. U.S. Pat. No. 3,203,822 and British Pat. No. 950,502 disclose the use of styrene-butadiene copolymers. German Pat. No. 20 03 702 uses a mixture of an epoxy-modified alkyd resin and unsaturated polyester resin. Published Japanese application No. Sho-47 (1972) 4951 discloses heat-setting butadiene polymers with an average molecular weight of from 1,000 to 5,000, in particular, low-molecular, atactic 1,2-polybutadienes with a content of about 90% of 1,2-compounds.
According to the state of the art of intermediate layers, these substances are used in the form of solutions in organic solvents such as white spirits, xylene, chlorohydrocarbons, ketones or esters. Expensive protective equipment is required both for people and for the environment, especially too as regards possible powder explosions, when working with these combustible solvents.